Carbon fibers are one of materials which are used in various applications as composite materials together with resins because the carbon fibers enhance the strength of the resins and, simultaneously therewith, the resins protect the carbon fibers from brittle fracture. However, the carbon fibers have had a problem in that since the carbon fibers, which are usually used in a form of fiber bundles constituted by a large number of filaments, have a low elongation, fluffing and filament fiber breakages tends to be occurred due to mechanical friction or the like especially during processing. Hence, a technique generally employed in using carbon fibers is to apply a sizing agent to the surface of the carbon fibers in order to improve the handleability of the carbon fiber bundles when using the carbon fibers. The use of a sizing agent is for enhancing bundling properties to improve handleability or for reducing the occurrence of fluffing or fiber breakages in the carbon fibers and simultaneously enhancing adhesion between the carbon fibers and resins to maximize the reinforcing effect of the carbon fibers.
For example, patent document 1 discloses a method in which in the case where a conventional epoxy resin is used as a matrix resin, a sizing agent based on an epoxy resin is used in order to improve interfacial bonding strength. Patent document 2 discloses a mehtod in which in the case of using polypropylene, which is a thermoplastic resin, as a matrix resin, a sizing agent based on an acid-modified polyolefin is applied.
However, use of the conventional sizing agents have had a problem in that even if the epoxy-based sizing agent or the thermoplastic-resin-based sizing agent is used, a texture of the carbon fiber bundles tend to be hardened although an improvement in interfacial bonding strength is brought about, and handleability and processability are considerably reduced. As a result, composites finally obtained have insufficient properties. Namely, the conventional sizing agents are insufficient in processability especially for cutting or opening, although effective in improving interfacial bonding strength. Furthermore, the epoxy-based sizing agent, which is in use most extensively, has had a problem in that although this sizing agent may be satisfactory in the case where the matrix resin to be reinforced by the carbon fiber bundles is a thermosetting resin, high bonding strength cannot be obtained in the case of using thermoplastic resins because such resins generally have poor compatibility with the sizing agent.
Meanwhile, patent document 3 discloses a technique in which a water-soluble polyamide resin is adhered to carbon fibers. This technique, however, is disadvantageous in that since the polyamide resin is water-soluble, the sized carbon fiber bundles inevitably have high hygroscopicity and are unusable in applications where the fiber bundles as such are passed through long steps. Namely, there has been a serious drawback that the sized carbon fiber bundles are usable in a limited form, i.e., the sized carbon fiber bundles are chopped, immediately impregnated with a resin, and formed into resin pellets, which are easy to dry.
Furthermore, in these conventional methods, there has been a problem in that the carbon fibers have too high bundling properties especially in fields where carbon fiber bundles in a tape form are opened and used. The individual single fibers in the carbon fiber bundles are not evenly dispersed in a composite material, resulting in an insufficient reinforcing effect. This problem has been serious especially in the case where carbon fiber bundles in a tape form are opened, separated, and cut, and the cut carbon fibers are randomly applied and impregnated with a resin, as in the case of random mats.
There has been a desire for a development in a surface treatment method of a carbon fiber bundle suitable for carbon-fiber composite materials, in particular, for random mats in which fiber bundles which are widened and opened are used.